Metallic effect pigments provide applications, such as paints and coatings, for example, with lustrous, brilliant effects, and fulfill functional requirements.
A key requirement of metallic effect pigments is the directed reflection of light at parallel-oriented pigment platelets. The peculiarity of applications with this kind of pigmentation is the pronounced angular dependence of the optical impression they feature; in other words, as the viewing angle changes, there are also changes in the lightness and, occasionally, in the color shade of the application as well.
Powder coatings are finding continually increasing use as solid and solvent-free coating materials in industrial mass production for the coating of electrically conductive and temperature-stable materials. The powder coatings, which are used as a primer or one-coat topcoat, are almost completely recyclable.
The powder coatings, which are eco-friendly and have diverse possible uses, comprise binders, pigments, fillers, and crosslinkers, and optionally additives as well.
Powder coatings are present in a finely divided form, and are generally applied electrostatically to various substrates and cured by baking or by radiation energy.
For the production of powder coatings, in a conventional mixing method, the raw materials for the coating, optionally after premixing in a solids mixer, are introduced into an extruder and homogenized in the melt at 80 to 140° C. The extrudate discharged from the extruder, cooled, and comminuted is subjected to an intense milling operation until the desired particle size is present.
For the pigmentation of powder coatings use is made, in addition to commercial chromatic pigments, of effect pigments produced by conventional ball mill grinding, such as, for example, platelet-shaped metallic effect pigments made of aluminum, copper, copper-zinc alloys or zinc.
The use of commercial platelet-shaped metallic effect pigments in powder coatings produced by mixing methods is problematic in that the shearing forces which act on the pigment platelets in the course of the extrusion and grinding operation can result in damage to or destruction of the pigment platelets, thereby causing negative impairment of, in particular, the gloss, and hence also of the optical qualities of the applications pigmented with these metallic effect pigments. This may go as far as the complete disappearance of the typical optical properties of the metallic effect pigments (brilliance, luster, flop, etc.) in the powder coating.
In order to prevent this, for example, the effect pigments used to pigment powder coatings are not mixed into the base powder coating until after the grinding procedure. A significant disadvantage of this powder coating production method, which is known as the dry-blend method, is the possible separation of pigment and powder coating during application of the coating material, owing to the different charging characteristics of the individual coating constituents. The consequence of this depletion or accumulation of pigment in the course of powder coating application is an irregular optical effect in the coated article. Moreover, the separation of pigment and binder makes it impossible fully to recover and re-use the environmentally damaging “overspray”, as it is called.
A further method for powder coating production is that known as the bonding method, in which the pigment is fixed to the particles of the basecoat by heating. The production of bonding powder coatings of this kind that can be used for optical high-grade coatings, however, is relatively costly.
The powder coatings that are presently the most cost-effective are produced by means of mixing methods. For such methods, the pigments are mixed together with all of the other raw materials, extruded, and ground. With this powder coating production operation, there is no need for the otherwise necessary worksteps of “dry blending” and/or “bonding”.
Powder coatings produced by mixing methods are pigmented with metallic effect pigments using, for example, dust-free gold-bronze and aluminum pigment preparations, which are traded commercially under the name “PowderSafe®” by ECKART GmbH, 91235 Velden. Although the one-coat finishes pigmented with these platelet-shaped metallic effect pigments have a very good metallic appearance, they are not sufficiently abrasion-stable for specific use purposes. Also the applications pigmented with these commercial metallic effect pigments cannot be additionally protected from mechanical and/or chemical influences by a clearcoat coating, as certain outdoor applications require. The reason for this is that the metallic effect pigments introduced in the powder coating have substantial leafing properties—that is, during the baking procedure, the pigment platelets float in the coating film and undergo alignment and ordering in the region of the film surface. These pigments thus then prevent effective attachment of the clearcoat to the basecoat, meaning that the powder coating is no longer resistant to abrasion.
For powder coating production by the bonding or dry-blend method, moreover, a large number of surface-coated/-modified effect pigments are used. These commercial pigments are, however, not resistant to damage and/or destruction due to the shearing forces that occur in the course of extrusion/grinding.
Effect pigments of this kind are traded, for example, by Merck under the name Iriodin®. These pearlescent pigments comprise mica platelets coated with metal oxides.
The Merck company also has surface-modified pearlescent pigments on the market that are coated with a polymer compound and are described in DE-A 43 17 019, for example.
Also employed for the pigmentation of powder coatings are coated Al2O3 platelets, bismuth oxychloride (BiOCl), aluminum flakes, Variochrom® or Paliochrom® pigments from BASF, LCP pigments (liquid crystal polymer pigments), and coated glass flakes or multilayer pigments.
Also known from EP 1 174 474 B1 is the use of SiO2 platelets or aluminum flakes coated with low molecular mass polyethylene or polypropylene.
In contrast, EP 1 558 684 B1 relates to a silane-modified pigment composition for use in metalized paints, printing inks, and plastics material. It is produced by grinding atomized aluminum powder by the known Hall process in the presence of silane instead of the fatty acids typically employed in that milling process. These aluminum effect pigments can be used in both aqueous and solventborne coating systems, on account of their improved corrosion resistance. The optical pigment properties are comparable with those of aluminum effect pigments produced by the conventional wet milling process.
Moreover, EP 1 084 198 B1 describes effect pigments with surface-modified orientation assistants. The orientation assistant, which is present in monomeric or polymeric form, carries at least two different functional groups, which are separated from one another by a spacer. One of the functional groups is attached chemically to the pigment, while the other is able to react, for example, with the binder of the pigment-surrounding varnish in a kind of crosslinking reaction and hence to contribute to the stabilization of the effect pigment with nonleafing quality.
DE 10 2005 037 611 A1 discloses metallic effect pigments with a hybrid inorganic/organic layer, possessing not only high mechanical stability but also good gassing stability. For this purpose, organic oligomers and/or polymers are joined to an inorganic network consisting of inorganic oxide components, the join being at least partly covalent via network formers. The network formers may inter alia be organo-functional silanes. The inorganic oxide component is constructed—when SiO2 is present—from, for example, tetraalkoxy-silanes. The application of a hybrid inorganic/organic layer requires a somewhat more complicated control of the process than when applying a coating composed of just one component.
EP 1 619 222 A1 discloses aluminum effect pigments having a silane-modified molybdenum- and silicon-oxide coating for water-based coating systems.
EP 1 655 349 A1 relates to recoatable effect powder coatings for good attachment of the clearcoat. These effect powder coatings comprise effect pigments which have been enveloped with a fluorine-containing polymer coating, but which do not afford adequate protection against destruction of the pigments under a shearing load. These pigments, therefore, can be incorporated only by the dry-blend or bonding method in the course of powder coating production.
JP 2003213157A discloses a metallic pigment for a powder coating composition with a high metallic luster. This aluminum pigment, which can be employed in single-coat or multicoat powder coating finishes, is coated with at least one resin component containing a fluorinated alkyl group. The coated aluminum effect pigments disclosed therein are employed in the powder coating by means of dry-blending or by bonding.
Further pigment preparations which, as well as effect pigments and other ingredients, also contain surface-active substances, such as alkylsilanes, for example, are described in DE 10 046 152A1, EP 1 104 447 B1, and EP 1 200 527 B1.
EP 0 955 344 A2 discloses organosiloxane-modified fillers which are used as additives to adhesives, sealants, polymer compositions, etc.
EP 1 076 266 A1 discloses a black toner which comprises particles of hematite or of iron oxide, the particles being provided first with a layer of organosilane and polysiloxane and thereafter with a carbon black layer.
US 2003/0161805 A1 discloses a coated powder for use in cosmetics. The powder may constitute a multiplicity of color pigments coated with a polysiloxane having basic groups, more particularly amino groups. The ratio of siloxy groups without a basic group to siloxy groups with a basic group is situated in the range from 5:1 to 1:5 and is preferably 1:1.
EP 1 690 884 A1 discloses organosilane-modified poly-siloxanes for surface coating, of fillers and pigments, for example, such as titanium dioxide pigments, which then, by virtue of their hydrophobic quality, can be incorporated effectively into plastics.